1. Field of the Invention
This invention relates to apparatus and methods for steering subterranean drilling assemblies and more particularly, but not by way of limitation, to apparatus and methods for deflecting the drilling direction of a rotary drill string and drill bit into and through a borehole wall.
2. Setting of the Invention
In order to enhance the recovery of subterranean fluids, such as oil and gas, it is sometimes desirable to orient the direction of the well bore or borehole. In an oil producing formation or strata which has limited vertical depth and relatively greater horizontal extent with respect to the surface of the earth, a borehole which extends horizontally through the oil producing formation may be more productive than one extending vertically. In order to create an inclined, highly deviated or horizontal borehole, it is necessary to steer the drilling bit at the end of the drill string from a generally vertical orientation to a lateral or horizontal orientation.
In steering a drill bit and drill string from a vertical orientation to a horizontal or other non-vertical orientation, it is necessary to deflect or side track the drill bit from the generally vertical borehole to a drilling direction inclined to the wall of the borehole. This initial step is also known as "kicking off" the drill bit and drill string. It is typical to first drill a vertical borehole and then attempt to deflect the drill bit and drill string by some means thereby causing it to drill through the wall of the existing borehole.
When drilling a deviated wellbore from a cased well, one must first cut and remove a section of the casing. Once the casing is removed, an opening is provided for the drill bit to pass through and into the cement surrounding the casing and then out into the formation. Due to irregularities in the cementing process, the casing may not be centered within the wellbore; further, the cement may not fully surround the casing. Thus, the actual distance between the sectioned casing and the material to be drilled (i.e., cement and/or formation material) is not known.
The drilling assembly (drill bit and drill string) is usually very flexible so it is partially unsupported as it spans the gap and thus is difficult to control, particularly during the "kick off" activities when the drill bit is just starting to drill into the borehole wall. This lack of control often necessitates the time consuming additional step of drilling a short pilot hole with a more easily controlled, nonflexible drilling assembly prior to using the flexible drilling assembly.
Another problem with the prior whipstocks is that the maximum deflection of the drill bit from the axis of the borehole ("exit angle") is in the range of about four degrees or less. As a result of this limitation, the total footage required to attain a horizontal trajectory and the amount of borehole casing that must be removed are much larger than with a greater exit angle.
Another problem with the prior whipstocks is that there is considerable difficulty in retracting the skewed or curved drill string and drill bit from the skewed borehole because the shoulders and other abrupt surface-changes in the drill string and drill bit hang up on the whipstock as they are pulled across the gap.
Therefore, there is a need for an apparatus and method of deflecting the drilling direction of a rotary drill string and drill bit through a borehole wall which will support the drill bit and drill string throughout its transition from the generally vertical borehole into the borehole wall, which will reduce the axial length of casing which must be removed to allow deflection through the borehole wall, which will reduce the likelyhood of snagging when retracting the drill bit and drill string from the borehole wall, which will eliminate the need for drilling a pilot hole with a rigid drilling assembly prior to running the flexible drilling assembly, which will provide greater control of kick off direction and angle, and which will reduce the total drilling footage required to attain a horizontal or other desired nonvertical drilling trajectory. It is contemplated that such an apparatus and method will allow more accurate steering of subterranean drilling assemblies and result in higher production rates of oil and gas.